This invention relates to the detection of the level of a substance in a closed tank. In particular, this invention is an improvement on the art of thermal level sensing devices, in that, it is an active integral unit that provides a continuous indication of level in a closed vessel.
One of the many techniques used to provide information on the level of a substance in a closed vessel with a liquid and a gas is based on the differences beween the fluids thermal characteristics. The thermal characteristics of fluids used to infer the level of the liquid in a tank are either temperature or thermal conductivity. Temperature probes measure the vertical temperature profile in the vessel and this information is used to infer the level of liquid in the vessel. Most systems based on the difference in thermal conductivities between two fluids are employed as fixed-point level sensors. These systems generally consist of electrically heated thermistors or thermocouples inserted into a vessel. In the case of thermistors, the temperature and, consequently, their electrical resistance, increase as the thermal conductivity of the fluid, at the level of a given thermistor, in which it is immersed in decreases. This increase in temperature is then used to infer the lack of fluid at the level of the thermistor evidenced by an increasing temperature value. In those systems that utilize thermocouples, the electrical heat is supplied by a separate electrical resistance heater juxtaposed next to in contact with the thermocouples which heats the thermocouples to a given equilibrium temperature. When the conductivity of the fluid surrounding the location of the thermocouple changes there is a corresponding increase in the millivolt output of the thermocouple. This change in the output of the thermocouple is used to indicate the level of the fluid in a tank.
U.S. Pat. No. 4,418,035, entitled "Coolant Condition Monitor For Nuclear Power Reactors," issued in 1981 to R. D. Smith consisted of a plurality of sheathed thermocouples arranged circumferential around a sheathed resistance heater, all of which were inserted in a pair of concentric steel tubes. The system being mechanically deformed such that all components are in good mechanical contact with each other. This conductivity monitor, because of its design can only detect the level of the fluid in which it is immersed at distinct locations along the monolithic unit. These devices are well suited as point level detectors for liquid-vapor interfaces, as the thermal conductivity of liquids is markedly higher than that of vapors. However, they can not provide a continuous indication of the level in a closed system.
The U.S. Pat. No. 4,418,035 provides information about the location of an interface between a fluid and the vapor above it at discrete locations. It should be noted, however, that the ability of the invention to resolve the location is limited by the number of sensors positioned latitudinally along the cylindrical probe. Level measuring devices which use resistance thermometers have this same limitation. It is desirable to overcome this drawback by providing a cylindrical level sensor which provides a continuous indication of the position of the interface between a fluid and a vapor, a solid and air or two fluids with different thermal conductivities. Level sensors of this type are useful when it is necessary to precisely control the level of a substance in a closed tank without recourse to the use of a cylindrical level indicator with a multiplicity of temperature sensors arrayed in an axial direction either exposed to the fluid or protected from it with a protective metallic sheath.
It is therefore an object of the present invention to provide an improved detector of fluid level in a closed tank.
It is a further object of the present invention to provide a continuous detector of level in a vessel. Other objects will be apparent in the course of a detailed description of the invention.